Vibrational studies onCO2up to 40 GPa by Raman spectroscopy at room temperature

Raman spectra of ${\mathrm{CO}}_2$ were obtained under hydrostatic (in ${\mathrm{H}\mathrm{e}-\mathrm{N}}_2$) and nonhydrostatic (pure ${\mathrm{CO}}_2$) pressures up to 40 GPa at room temperature. The pressure-induced changes of the librational spectra suggest that solid ${\mathrm{CO}}_2$ does not transform directly from phase I (cubic $\mathrm{Pa}3\mathrm{}$) to phase III (orthorhombic $\mathrm{Cmca}$). An intermediate phase IV, which may involve only minor modifications of phase I, is formed prior to the transition to $\mathrm{Cmca}$ structure. Phases IV and III coexist over some pressure range. The pressure shifts of all four Raman-active librons of phase III were determined over a wide pressure range and are compared to previous theoretical results. Splittings in the ${\nu }^{-}$ and ${\nu }^{+}$ modes are observed in the stability field of phase I below 10 GPa. The frequency of the ${\nu }^{+}$ mode decreased slightly whereas the frequency of the ${\nu }^{-}$ mode increased slightly at the transition to phase III. The liquid-to-solid transition at 0.6 GPa was confirmed.